Help subtracting these fractions please

[chriscarson]

Homework Statement

Subtracting mixed numbers

Relevant Equations

Least common multiple

I really can t find my mistake here 17/20 is the proper answer

Thanks

 

[berkeman]

When you put the 3 fractions over the common denominator of 20, it looks like you did not convert the numerators correctly.

First step is to put the front whole number of each quantity into the numerator of each fraction without changing the denominator yet. So $5\frac{3}{4} = \frac{?}{4}$

Then after you have converted each of the 3 quantities to pure fractions, go ahead and multiply to put them over the common denominator of 20. Does that work for you?

 

[DaveC426913]

[ DUPE ]

 

[berkeman]

[ DUPE ]

(Great minds think alike!)

 

[DaveC426913]

Heh. I was commenting on what appears to be a different error: -3/20 magically becoming 17/20.

But I see what the OP is doing. He's actually doing 1 plus -3/20, which does equal 17/20.

The OP is trying to solve the problem by splitting the number into whole and fractions and dealing with them separately:

= (5 -1 -3) + (3/4 -2/5 -1/2)

= 1 + (etc.)This actually does result in the right answer (and frankly, it's easier math).

 

[chriscarson]

When you put the 3 fractions over the common denominator of 20, it looks like you did not convert the numerators correctly.

First step is to put the front whole number of each quantity into the numerator of each fraction without changing the denominator yet. So $5\frac{3}{4} = \frac{?}{4}$

Then after you have converted each of the 3 quantities to pure fractions, go ahead and multiply to put them over the common denominator of 20. Does that work for you?

I will try to understand it like you are showing me , thanks

 

[chriscarson]

[ DUPE ]

I think you mean I make magics lol

 

[chriscarson]

When you put the 3 fractions over the common denominator of 20, it looks like you did not convert the numerators correctly.

First step is to put the front whole number of each quantity into the numerator of each fraction without changing the denominator yet. So $5\frac{3}{4} = \frac{?}{4}$

Then after you have converted each of the 3 quantities to pure fractions, go ahead and multiply to put them over the common denominator of 20. Does that work for you?

First step is to put the front whole number of each quantity into the numerator of each fraction ? can you tell how please ?

 

[DaveC426913]

First step is to put the front whole number of each quantity into the numerator of each fraction ? can you tell how please ?

Let's start simply.

For the number 1 1/2, what would that be as a fraction only? How many halves are there in and and a half?

 

[chriscarson]

if you can post a picture with a written work like mine should help me better thanks

 

[chriscarson]

Let's start simply.

For the number 1 1/2, what would that be as a fraction only? How many halves are there in and and a half?

3/2 ?

 

[DaveC426913]

if you can post a picture with a written work like mine should help me better thanks

Sorry. You're supposed to do the work, not us.

 

[DaveC426913]

3/2 ?

OK, how did you get that from 1 1/2?

 

[chriscarson]

OK, how did you get that from 1 1/2?

2*1+1

 

[DaveC426913]

2*1+1

Perfect.
Can you do the same for 5 3/4?

 

[chriscarson]

Perfect.
Can you do the same for 5 3/4?

23/4

 

[DaveC426913]

Perfect. Now the other two in your problem.

 

[chriscarson]

Perfect. Now the other two in your problem.

7/5 oh so my mistake was that i was making /20 ?

 

[DaveC426913]

7/5 oh so my mistake was that i was making /20 ?

You're going to do that next.

Show all your work so far.

 

[chriscarson]

 

[chriscarson]

You're going to do that next.

Show all your work so far.

 

[berkeman]

Yes, now put them over the common denominator of 20, do the resulting subtraction, and you have the correct answer!

 

[chriscarson]

Yes, now put them over the common denominator of 20, do the resulting subtraction, and you have the correct answer!

don t ask me why but I tried this already but somehow had different answer so I gave up
well thank you all

 

[Ibix]

don t ask me why but I tried this already but somehow had different answer so I gave up
well thank you all

It's very, very easy to make mistakes with this kind of work, no matter how experienced you are. If you think you know how to do something but get the wrong answer, turn the page and try again. There's a fair chance you just wrote a number wrong somewhere...

 

[chriscarson]

Yes, now put them over the common denominator of 20, do the resulting subtraction, and you have the correct answer!

The thing is that here it worked out without make them as fractions

 

[Ibix]

View attachment 264583

The thing is that here it worked out without make them as fractions

And this time you got one and minus three twentieths, right? Which is 17/20. So it looks like you actually did the maths correctly - it's just that with mixed fractions you are (by convention) not allowed to have a negative fraction. So you can write $1\frac 12$ (meaning one and a half), but $2\frac{-1}2$ (meaning two minus a half) is just something we don't write. If we get it, we either write it as $\frac 32$ or $1\frac 12$.

 

[chriscarson]

And this time you got one and minus three twentieths, right? Which is 17/20. So it looks like you actually did the maths correctly - it's just that with mixed fractions you are (by convention) not allowed to have a negative fraction. So you can write $1\frac 12$ (meaning one and a half), but $2\frac{-1}2$ (meaning two minus a half) is just something we don't write. If we get it, we either write it as $\frac 32$ or $1\frac 12$.

View attachment 264583 that this was an addition it was a different story you mean ?

 

[Ibix]

Let's take a simple case: $3\frac 13-1\frac 16$. Your original method would be to think $3-1=2$ and $\frac 13-\frac 16=\frac 16$ to give an answer of $2\frac 16$ - right?

Now let's try another example: $4\frac 12-2\frac 23$. Your original method would be to think $4-2=2$ and $\frac 12-\frac 23=-\frac 16$, to give an answer of $2\frac{-1}6$. That's not wrong - it's just that the convention for writing numbers in this mixed form is that the fraction is always positive and less than one. This one is negative. All you need to do is work out that $2-\frac 16=1\frac 56$.

You can have a similar problem using your methodology to add, if the fractions add up to more than one. For example $1\frac 23+2\frac 12=3\frac 76$. But seven sixths is more than one, so you'd write $3\frac 76$ as $4\frac 16$.

 

[FactChecker]

Your final answer is not how you should ever write a mixed number. You can not simply concatenate the integer +1 and the fraction -3/20 directly to a proper mixed number. The integer part and the fractional part must have the same sign. Change that integer, 1, into a fraction and subtract the fraction numerators to get a valid answer.

 

[chriscarson]

Let's take a simple case: $3\frac 13-1\frac 16$. Your original method would be to think $3-1=2$ and $\frac 13-\frac 16=\frac 16$ to give an answer of $2\frac 16$ - right?

Now let's try another example: $4\frac 12-2\frac 23$. Your original method would be to think $4-2=2$ and $\frac 12-\frac 23=-\frac 16$, to give an answer of $2\frac{-1}6$. That's not wrong - it's just that the convention for writing numbers in this mixed form is that the fraction is always positive and less than one. This one is negative. All you need to do is work out that $2-\frac 16=1\frac 56$.

You can have a similar problem using your methodology to add, if the fractions add up to more than one. For example $1\frac 23+2\frac 12=3\frac 76$. But seven sixths is more than one, so you'd write $3\frac 76$ as $4\frac 16$.

I think i got it now . Never thought that the denominator is the last number that the numerator have to be the highest value of and than you add one whole.

 

[chriscarson]

Your final answer is not how you should ever write a mixed number. You can not simply concatenate the integer +1 and the fraction -3/20 directly to a proper mixed number. The integer part and the fractional part must have the same sign. Change that integer, 1, into a fraction and subtract the fraction numerators to get a valid answer.

Integer you mean the whole number ?

 

[FactChecker]

Integer you mean the whole number ?

Yes

 

[chriscarson]

Yes

So you make 20 × 1 - 3 = 17 then / 20 .
Good looks more clear now for me .
Thanks